updated 2019

The powersupply unit (PSU) of your PC is quite important, as one with bad quality can give unstable PC performance.
There are so many brands and models, and for newbees it can be hard to find out what to choose...

Here are some of the important things I have experienced:
   The first thing that should be OK is quality, quality and quality.
   The price is strongly related to quality.
   A bigger blower means less noisy.
   High power rating is not always better, as efficiency and stability is worse when you take little power from it.
   Good efficiency means less dissipated power, less need for cooling and thus less blower noise as it automatically adjust its speed.
   Some brands that were good earlier now make bad powersupplies.
   Some powersupplies of a specific brand are made by other companies as OEM.
   Information of the shop or manufacturer can be misleading.
   Many forums that discuss powersupplies don't have usable info.
   NB here is info about the standard connector that connects to your motherboard, and my experience with a non-standard version of it: power supply connector

About the all-important quality:

- Many powersupplies use cheap capacitors that result in short lifetime, instability and many headaches.
- There is a classification of efficiency, and this is very often directly related to the quality of its components and thus total quality.
- Some powersupplies use thin wires in order to make it cheaper. This results in voltage drop and instability.
- Some powersupplies are "modular", which means that it has connectors in its case where you can connect the cables you want and thus don't have the spaghetti of unused cables. But when the quality of that connector isn't super good, it will introduce extra voltage drop and possibilities of failure. So it is often best not to have the modularity.!

There is one forum that has really good information. They test powersupllies, take them apart and review the quality of the internal components. I agree with most of their technical conclusions. link to hardocp powersupply reviews

One of the things you will run into when investigation powersupplies is the 80+ or "80plus" ranking of them.
Here is a link to a good explanation of the "80plus" classification:
The ranking is mostly about efficiency, but my experience is that anything less than silver is bad quality.
One of the reasons for that is probably that low efficiency will result in more power dissipated in the components inside the powersupply and thus more chance for breakdown and shorter lifetime.

When building the PC called "Tuesday", I needed a long-life powersupply of approx. 500W.
I investigated a lot and ended up buying the Corsair TX550.
It has no negative reviews, uses good capacitors and is not super expensive.
The cheaper powersupplies like Corsair FS550, CX550 have many bad issues, and the best which probably is the RM550 is quite expensive.
Corsair CP-9020133-UK TX550M 550 W 80+ Gold Power Supply Unit 66.99
This powersupply is "semi-modular", which means that the important wires are fixed, and some of the auxilary like sata supply cables have a connector in the powersupply case and can be removed when not needed.

Also XFX has some good powersupplies, and some of the gold Coolermaster.
Seasonic and NorthQ also used to make good powersupplies, but many of them are now crap.

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